Multimode optical fiber is used in a wide variety of short range (<100 m) communications applications. The design of these fibers is increasingly specialized, and there is demand for higher bandwidth communications over longer ranges. Two quantities of interest in multimode fibers are modal dispersion, which is a measure of the relative speeds of the modes in the fibers, and modal spacing, which is a measure of the relative wavelengths of the modes in the fiber. The relative speeds of the modes in the fiber are characterized by a quantity called the relative group-index, and the relative wavelengths of the modes are characterized by a quantity called the relative effective-index. These two relative index values are important fiber design parameters, but they are difficult to determine and have largely been theoretical in nature.
In U.S. Pat. No. 7,330,245, the inventor describes how to measure the relative group index and effective-index of the two modes propagating in Polarization Maintaining (PM) fiber. In the case of PM fiber, the two modes have the same distribution and occupy the same space but have different polarizations. In contrast, the modes in a multimode fiber occupy the same space, but their distributions in this space differ significantly and are orthogonal functions. As a result, it has been assumed that different modes in multimode fiber would have different uncorrelated Rayleigh scatter patterns because the scatter centers are assumed to be distributed three dimensionally through the core. Given these assumptions, the expectation has been that cross correlations of the Rayleigh scatter patterns for the different modes in the multimode fiber would not provide useful results. Another concern is Rayleigh scatter measurements from multimode fiber are weaker (lower amplitude) than those from single mode fiber or PM fiber. Consequently, short segment Rayleigh scatter data correlations typically used for PM fiber produce low level peaks when used for multimode fiber, which makes analysis of correlations for multimode fiber difficult.